Radioactive logging apparatus



SePt- 7 1954 H. J. Dl GIOVANNI ETAL 2,688,703

RADIOACTIVE LOGGING APPARATUS Filed May 26, 1952 3 Sheets-Sheet lINVENTORS H060 d D/G/I//QN/V/ ROBERT 7.' GRA V550/V BY ALFRED H. YL/

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ATTRNEY Sept. 7, 1954 H J. Dl GIOVANNI ET AL 2,688,703

RADIOACTIVE LOGGING APPARATUS Filed May 25, 1952 5 Sheets-Sheet 2 IN VEN TORS H060 d DIG/UVA/V/V/ ROBERT 7.i GAVESA/ BY ALFRED H. YL/

,MM/4 4M ATTORNEY SePt- 7 1954 H. J. Dl GIOVANNI ET AL 2,688,703

RADIOACTIVE LOGGING APPARATUS 3 Sheets-Sheet 3 Filed May 26, 1952 FIG.3.

ATTORNEY Patented Sept. 7, 1954 RADIOACTIVE LOGGING APPARATUS Hugo J. DiGiovanni, New York, Robert T. Graveson, Yonkers, and Alfred H. Yoli, NewYork,

N. Y., assignors, by mesne assignments, to the United States of Americaas represented by the United States Atomic Energy Commission ApplicationMay 26, 1952, Serial No. 290,048

(Cl. Z50-'71) 6 Claims.

The present invention relates to apparatus for detecting and measuringthe radioactivity of deposits in the earth. Originally, in prospectingfor radioactive material in various areas, it was the usual practice todrill a hole down through the deposit and bring out the drilled core ofthe material which was chemically analyzed for radioactive material suchas uranium. More recently, instruments have been developed which may beinserted into the drilled hole on the end of a long cable. Theconventional instrument includes a radiation counter tube in one endthereof which responds to any radioactivity present in the deposit andemits pulses that are transmitted through the cable to recordingapparatus above the surface of the earth. By noting the variation of therate of occurrence of the emitted pulses with the depth of the insertionin the drilled hole, it is possible to locate the radioactive depositsin the hole.

One of the problems involved in using such conventional instruments isthe length of the radiation counter tube. The diameter of this tube islimited by the diameter of the drilled hole. Therefore, to increase thesensitive counting volume of the tube, the length of the tube must beincreased. If the length of the tube is small, its sensitive countingvolume will necessarily be small so that the counting time must belengthened to achieve the same statistical accuracy. If the tube is madelong, it is difficult to determine the extent of the radioactivedeposit, which in some cases is not more than one inch in thicknesswhile in other cases it may extend many feet in depth. One solution isto use a number of different sized counter tubes mounted in subsurfaceor probe units, each unit containing a different length radiationcounter tube. The longest probe, using a tube approximately one foot inlength, is initially inserted into the drilled hole. lf any radiation isdetected, the approximate location is noted and the probe removed. Asmaller sized probe is then inserted. By successively decreasing thesize of the counter tubes, it is possible to locate the extent of theradioactive deposit along the drill hole with a reasonable degree ofaccuracy.

The apparatus of the present invention does not require this lengthyprocedure, as it includes a scintillation counter which is very small inphysical size but which responds to low as well as high levels ofradioactivity. Another difliculty encountered with conventionalinstruments is the determination of the energy of the radiation emittedin the deposit. Since the conventional instruments usecounter tubesoperating in the Geiger region, the output pulse of the tube will alwayshave the same amplitude regardless of the radiation energy. With thescintillation counter of the present instrument, the amplitude of theoutput pulses bears a direct relation to the energy of the impingingradiation. Another advantage of the present instrument is the improvedstructural features of its probe unit which smplines operatingprocedures While simultaneously decreasing maintenance time.

More particularly, the present invention relates to radiation measuringapparatus, for use with drill hole boring apparatus, which includes asubsurface unit and a surface unit connected by a coaxial cable. Thesubsurface unit includes means responsive to the impinging radiation fordeveloping voltage pulses proportional to the number and energy thereof,means for amplifying the voltage pulses and developing output pulses fortransmission along the coaxial cable to the surface unit. The surfaceunit includes means for amplifying the output pulses, oscillating meansresponsive to the amplified output pulses for developing one rectangularpulse of predetermined height and width for every amplified outputpulse, means for varying the height of the rectangular pulses, a,ratemeter circuit responsive to the rectangular pulses for detecting therate of arrival and amplitude thereof and power supply means forsupplying operating voltages to the subsurface unit along the coaxialcable and simultaneously to the surface unit` It is accordingly anobject of the present invention to provide a new and improved radiationmeasuring apparatus for use with drill hole boring apparatus.

A second object of the invention is to provide a new and improvedradiation measuring apparatus for use with drill hole boring apparatuswhich provides output pulses proportional to the energy of the incidentradiation.

Still another object of the invention is to provide a new and improvedradiation measuring apparatus using a scintillation crystal.

A further object of the invention is to provide a new and improved probeunit for use with drill hole radioactive logging apparatus.

The many objects and advantages of the present invention may best beappreciated by reference to the accompanying drawings, the figures ofwhich illustrate apparatus incorporating a preferred embodiment of thepresent invention.

Fig. l is a block diagram of the various operating elements showing theinterconnections between the circuits of the apparatus.

Fig. 2 is a schematic wiring diagram of the subsurface probe unit.

Fig. 3 is a schematic wiring diagram of the circuits used in the surfaceunit.

Fig. 4 is a perspective View of the probe unit with its housing removedto show the interior constructional details.

Referring now to Fig. l, the relationship between the various circuitsof the apparatus will be described: All the power used for the surfaceunit and the subsurface unit of the apparatus may be derived from aconventional generator i9. One of the outputs of generator I8 energizesthe power supply 28 by means of conductor and ground connection l2. Theother output of generator I is connected tothe amplifier 39 by means ofconnector |3 and ground connection l2. This output from generator i9 istransmitted from amplifier 3i) to the probe unit 40 through a coaxialcable |5.

The probe unit 49 is the subsurface unit of the instrument and thepulses derived therefrom are transmitted to amplifier 38 of the surfaceunit by means of the coaxial cable I5. The amplified output pulses fromamplifier 30 are transmitted to univibrator 50 through. conductor andground connection i2. In turn, the univibrator output is applied to theinput of the ratemeter 65 on conductor le and the ratemeter output isindicated by Vacuum-tube voltmeter l?! through the conductor 9. Furtheroperating voltages for the surface unit are supplied from the powersupply 29 by vmeans of conductors 2|, 22 and 23. Power supply 20 is aconventional unit capable of providing both positive and negative vacuumtube operating potentials. The various components of the surface unitincluding the generator i8 may conveniently be mounted on mobileautomotive equipment which could also carry a reel for the coaxial cableI6.

Referring now to Fig. 2, the operation of the subsurface probe unit willbe described. A scintillation crystal (not shown) is mounted on thephotosensitive face of the photomultiplier tube indicated generally bythe arrow 25. Voltage dropping resistors 26 are connected between theplurality of dynodes of the photomultiplier tube. rlhe last two dynodes2'! and 25 and the anode 29 are respectively connected to the groundconnection I2 through resistors 3|, 32 and 33. The last dynode 25 isalso directly connected to the control electrode of tube 55 by means ofconductor 36. The cathode of the tube 3d is connected to ground throughresistor 3i and the plate is coupled to the control electrode of tube 88through condenser 39. The control electrode of tube 38 is furtherconnected to ground by means of resistor 4| and the cathode is directlyconnected to ground through conductor 42.

The plate ofthe tube 38 is connected to the cathode of the tube 34through resistor 43 and is further coupled to the control electrode oftube 44 by means of condenser 16. Thev suppressor grid of the tube 44 isdirectly connected to the cathode. The control electrode and the cathodeare respectively connected to ground through resistors 4l and 48.Across. resistor it is connected a bypass condenser 49. The screen gridof tube 44 is connected to ground through condenser 5| and is furtherconnected to one side of the secondary winding 52 of transformer y53 bymeans of conductor 54. The other side of the secondary winding 52 isconnected on conductor 55 to the plate of the tube 44. The primarywinding 5l of the transformer 53 has one side coupled to ground throughcondenser 58 and its other side directly connected to the centerconductor of coaxial cable I6 by conductor 59.

Cable |6 is further connected to one side of primary winding 5| of highvoltage transformer 62 by conductor 63, and onel side of primary winding64 of filament transformer 66 by conductor 61; and to one side ofprimary winding 6B of low voltage transformer 69 by means of conductor1|. The other sides of the primary windings are all connected togetherand to ground by means of conductor 12. One side of the secondarywinding 73 of the transformer 62 is connected to the junction pointbetween condensers 14 and 16. The other side of secondary winding 13 isconnected to condenser 'I6 through rectifier l1 and to condenser 'I4through rectier '|8. The ljunction point between condenser 'I4 andrectiner '|8 is connected to the cathode of voltage regulator tube 79through resistor 8| and the junction point of condenser 'I6 andrectifier 'l1 is connected to the anode of tube 19 by conductor 82. Thecathode of tube 19 is further connected to the cathode 83 ofphotomultiplier tube 25 through resistor 8!! and conductor 86. The anodeof tube 19 is connected to the plate 29 of photomultiplier tube 25 bymeans of conductor 81 and is further connected to ground throughcondenser 88. Also connected from the anode of tube 19 to the cathode 83of tube 25 is a condenser 89.

One side of the secondary winding 9| of low voltage transformer 69 isdirectly connected to one plate of a double diode tube 92 and the otherside of secondary winding 9| is connected to the other plate of tube 92.The center tap of secondary winding 9| is connected to ground byconductor 93. The cathodes of tube 92 are connected together and arefurther connected by means of conductor 94 to an R-C filter 95comprising resistors 96 and condensers 91 and 98. The other side ofcondensers 91 and 98 are connected together and to ground by conductor99. The output of the nlter 95 is connected on conductor |0| to theplate of tube 34 through resistor |02, as well as to the plate of tube38 through resistor |03, and also to the screen grid and plate of tube44 through conductors |04 and 54 and secondary winding 52 respectively.The secondary winding of filament transformer 66 is connected across thefilaments |06 of the various tubes to provide proper heater potential.

In operation, low frequency alternating current voltage from generatorI0 is applied to the coaxial cable |5 and from there to the primarywindings of transformers 82, 66 and 69. Although the alternating currentvoltage is applied on conductor 59 to the primary winding 51 oftransformer 53, essentially no voltage is induced in the secondary ofthis transformer because the condenser 58 in series with the primarywinding acts as a virtual open circuit at low frequencies. Therefore,substantially all of the applied alternating current voltage iseffective in inducing voltages in the secondary windings of transformers62, 65 and 59.

The secondary voltage induced across the filaments |06 of the filamenttransformer 66 establishes the proper heater potentials for the varioustubes in the probe unit. The voltage induced in the secondary winding 9|of transformer 69 is rectied by the full wave rectifier action of tube92, smoothed out by the R-C filter network 95 and applied to the platesof tubes 34, 38 and 44 and the screen grid of tube 44. The

voltage induced in the secondary winding 13 of the transformer 62 isrectified and doubled by the voltage doubler circuit represented bycondensers 14 and 16 and rectiiiers 11 and 18. The output of thisvoltage doubler circuit is regulated by the tube 19 and applied to thephotomultiplier tube 25 between the cathode 83 and the plate 29.

The probe unit is now in condition for operation. Any radiationimpinging on the scintillation crystal will cause the crystal to emit aproportional number of light photons. These photons appearing at thephotosensitive cathode 83 will give rise to secondary electrons whichare successively amplified and applied from the last dynode 28 to thecontrol electrode of tube 34 on conductor 36. This pulse is positive andis ampliiied by preamplifier tube 34 giving rise to negative amplifiedpulse output from the plate thereof. The amplified pulse is applied tothe control electrode of tube 38 through condenser 39 for furtheramplification.

The output is taken from the plate of tube 38 and is fed back throughresistor 43 to the cathode of the tube 34 to provide degenerativefeedback. This increases the stability and the bandwidth of thepreamplier. The output from the plate of the tube 38 is further appliedto the control electrode of tube 44 through condenser 46. Tube 44provides power amplification for the applied pulse and the output fromthis tube is coupled to the coaxial cable I6 through the pulsetransformer 53. Since this output pulse represents only very highfrequencies, the condenser 58 in series with the primary winding 51 issubstantially a short circuit. On the other hand, the' primary windingsof the transformers 62, 66 and 69, to which the output pulse is alsoapplied, represent an eiTective open circuit thereto.

The transformer coupling between the power amplification stage and thesurface unit (transformer 53) also provides means for closely matchingthe impedance of the coaxial cable. This provides a maximum powertransfer between these units and results in highly eiiicient subsurfaceprobe.

Therefore, the function of the probe unit is to emit amplified outputpulses on coaxial cable |6 whose amplitude is proportional to the energyof the radiation impinging on the scintillation crysr tal in the mannerhereinabove described. The rate of occurrence of the outputpulses isdirectly proportional to the rate of radiation impingement.

Referring now to Fig. 3, the operation of the surface unit will be setforth. The coaxial cable |6 is connected to the variable transformer onconductor H2, through an ammeter H3. A voltmeter |4 is connected betweenthe conductor H2 and the ground connection i2. The coaxial cable is alsoconnected to one side of the primary winding H6 of transformer H1. Theother side of the primary winding is connected to ground throughcondenser H 8. Connected across the secondary winding H9 of transformerH1 are three parallel rectifiers |20. One side of the secondary windingI9 is connected to ground and the other side is connected to the controlelectrode of tube |2| through a network |22 comprising condenser |23 andtapped resistor bank |24. The cathode of the tube |2| is connected toground through cathode resistor |26 and bypass condenser |21. The plateis connected to the positive voltage conductor 2| through resistors |28and |29. The junction point of these Cil resistors is connected toground through condenser |3|. The plate of the tube |2| is furtherconnected through condenser |32 to an external coaxial terminal |33.This terminal is provided so that an. external stage of conventionalampliiication may be used if desired.

Normally, terminal |33 is directly connected to terminal |34 which inturn is connected to the control electrode of tube |36 by means ofconductor |1. The control electrode is further connected throughresistor |38 to the junction point of voltage divider resistors |39 and|4|. Resistors |39 and 54| are connected in series between the positivevoltage supply 22 and ground. Across resistor |4| is connected a bypasscondenser |42. The plate of tube |36 is connected to positive supply 2|through resistor |43 and is further connected to the control electrodeof tube |44 through a coupling condenser |46.

The cathodes of tube |36 and |44 are connected together and are furtherconnected to ground through resistor |41. The control electrode of tube|44 is connected to ground through resistor |49 and the plate isconnected to positive supply voltage 2| through plate resistor |49. Theplate of the tube |44 is also connected through a coupling condenser |5|to a terminal |52. Terminal |52 is connected to a second terminal |53and to ground by means of resistor |54. Terminals |52 and |53 areprovided for attachment to an oscilloscope if desired to View the pulsesat this point.

The plate of the tube |36 is further connected to the control electrodeof the tube |56 through the coupling network comprising condenser |51,potentiometer |58 and resistor |59. The plate of the tube |56 isconnected to the positive voltage supply 2| by means of anode resistorISI, as well as to ground through condenser |62 and is directlyconnected also to the screen grid. The suppressor grid of the tube |56is connected to the cathode by conductor |63' and the cathode in turn isconnected to ground through resistor |64. The output of tube |56 istaken from the cathode by means of conductor |'8 and applied to one sideof a tapped condenser bank |66.

The variable tap |61 of condenser bank |66 is mechanically coupled tovariable tap |68 of resistor bank |69. This is indicated by the dottedline |1|. The tap |61 is connected to the plate of diode |12 and thecathode of diode |13. The plate of diode |13 is directly connected tonegative voltage supply conductor 23. The cathode of the tube |12 isconnected to a variable network indicated generally by the numeral |14and comprising condensers |16 and |11 and tapped series resistors |18.The cathode of the tube |12 is also connected to ground throughcondenser |19 and to resistor bank |69 by means of conductor |8I.

The output of the network |14 is applied to the control electrode oftube |82 through conductor I9. The plate of the tube |82 is directlyconnected to its screen grid through conductor |93 and is furtherconnected to the positive voltage supply conductor 22 through resistor|84. Bypass condenser |86 is connected from the plate to ground. Theplate of the tube |82 is also directly connected to the plate and screengrid of tube |81 by conductor |86.

The control electrode of tube |81 is connected to ground on conductor|89 and the cathode is connected to the cathode of tube |82 throughresistor |9|, potentiometer |92 and resistor |93. The tap |94 of thepotentiometer |92 is connected to ground. The cathodes of these tubesare further connected through resistor |96, ammeter |91, and resistor|98. A connector jack |99 is connected across resistor |98 for purposesto be described hereinbelow.

In operation, the output pulses appearing on coaxial cable I6 areapplied to the amplifier 30 through transformer I |1. In series with theprimary winding IG of transformer |1 is condenser H6 which acts as aneffective short circuit with respect to the high frequency components ofthe input pulses. On the other hand, the winding of variable transformeris effectively an open circuit with respect to these pulses due to theimpedance of its winding.

Therefore, substantially all of the input pulses are effective ininducing voltages in the secondary winding ||9 of transformer ||1. Thesepulses are applied through network |22 to the control electrode of theamplifier tube |2|. The amplied output pulses of tube |2| appear at itsplate and are applied to coaxial terminal |33 through condenser |32.From this terminal they may be applied to a conventional amplifier ifdesired and the output of that amplifier impressed on coaxial terminal|34 or else these amplified output pulses may be directly applied toterminal |34 as illustrated.

Therefore, emerging from the amplifier 30 on conductor |1 are sharpamplified pulses whose amplitude is proportional to the energy of theradiation appearing in the drill hole and whose rate of occurrence isproportional to the impingement rate of the radiation. These pulses areapplied on conductor I1 to the control electrode of the first tube |36of the cathode-coupled univibrator comprising tubes |36 and |44. As iswell known in the art, a cathode-coupled univibrator circuit is anoscillator circuit which emits a rectangular pulse of predeterminedamplitude and width at a repetition rate controlled by the rate ofoccurrence of the applied input pulses.

In operation, tube |36 is normally in the conductive state due to thepotential on its control electrode determined by the voltage dividernetwork consisting of resistors |39, |4| and |38. When tube |36 isconducting, the potential drop across the cathode resistor |41 of theunivibrator circuit is sufliciently high to prevent tube |44 fromconducting. When the negative pulse appearing on conductor |1 isappliedto the control electrode of the tube |36, it will decreaseconduction in this tube and the amplied positive pulse appearing at itsplate will charge condenser |46. This will increase the potential at thecontrol electrode of the tube |44 causing it to conduct andsubstantially increase the voltage drop across cathode resistor |41. Theresult is to cut olf tube |35 and provide a positive rectangular pulseof predetermined height and width in accordance with the time constantsof the circuit. This positive rectangular pulse is applied throughcoupling condenser |51 to the control electrode of the cathode followertube |56 and the output from this tube is taken from its cathode onconductor |8.

When the condenser |46 at the control electrode of tube |44 hassufliciently discharged, the tubes will return to their normal stateswith tube |36 conducting and tube |44 non-conducting. This places theunivibrator circuit in condition to emit another positive rectangularpulse upon the arrival of a negative input pulse on conductor I1.Therefore, the output from the univibrator circuit 50 appears onconductor I8 and consists of positive rectangular pulses ofpredetermined height and width with a repetition rate determined by therate of occurrence of the applied pulses appearing on conductor l1.

The rectangular pulses appearing on conductor i8 are applied to thetapped condenser bank |66 of the ratemeter circuit 60. The position ofthe tap |61 determines the size of the condenser chosen from condenserbank |66 and serves to change the amplitude of the pulses applied to theplate of the diode |12. That is, the amplitude of these pulses isdetermined by the charging rate of the condenser in bank |66 coupledwith the internal impedance of the cathode follower tube |56. Thesepulses are applied to the plate of the diode |12 to cause this tube toconduct and charge the R-C network comprising the condenser |19 andtapped resistor bank |69.

The rate at which charged condenser |19 will discharge through theresistor chosen from bank |69 will depend on the relative magnitudes ofthese components. It can be seen that if these magnitudes are chosen sothat condenser |19 is not fully discharged before the arrival of thenext rectangular pulse on tube IIB, the condenser will maintain a steadycharge dependent upon the rate of arrival of the rectangular positivepulses.

Since the rate of arrival of these pulses is proportional to the rate ofoccurrence of the detected radiation in the scintillation crystal asdescribed hereinbefore, the voltage appearing across condenser |19 willbe proportional to this rate of occurrence. This Voltage is appliedthrough the tapped resistor bank |14 to the output conductor i9.Therefore, a voltage appears on output conductor |9 with an amplitudeproportional to the rate of occurrence of the detected radiation.

The pulses appearing on conductor |9 are applied to the controlelectrode of the tube |82 which is the first tube of a conventionalbridgetype vacuum-tube vcltmeter circuit 19. The application of thevoltage to the control electrode of the tube |82 unbalances the bridgecircuit and causes an unbalance current to flow through the ammeter |91.Therefore, the indication on the ammeter |91 is directly related to therate of occurrence of the detected radiation.

If it is desired to use a conventional recording voltmeter instrument,such as the Electronik Recorder manufactured by theMinneapolis-I-Ioneywell Regulator Co., Brown Instruments Division,Philadelphia, Pa., the instrument can be connected to the connector jack|99 to measure the voltage across the resistor |93 due to the unbalancecurrent.

The operation of the complete instrument will now be described. Thesubsurface probe unit 40 is attached to the surface unit of theapparatus by means of a long coaxial cable. This instrument is capableof operating with lengths of cable in the order of 1000 feet. The entirecable normally is kept on a reel, not shown, which can be convenientlymounted on a motor vehicle, not shown, with the rest of the surfaceunit. When it is desired to measure an earthen deposit forradioactivity, a hole is drilled at the desired locality withconventional drill hole borlng apparatus. The subsurface probe unit 40is then dropped down the length of the hole. The amount of cable thathas been played out from the reel can be determined by using aconventional odometer at the output of the reel. The output of generatormounted on the motor vehicle is connected across the variabletransformer of Fig. 3. The desired input current and voltage to theprobe unit 40 is established by varying transformer and reading ammeter||3 and voltmeter ||4.

As described hereinabove, this generated voltage is a comparatively lowfrequency voltage in the order of 400 cycles, so that condenser IIS inseries with primary ||6 of transformer ||1 is an effective open circuitto this voltage. Therefore, none of this alternating current voltageinduces a voltage in the secondary winding ||9 to disrupt the operationof the following circuits. The alternating current voltage is applied oncoaxial cable |6 to the subsurface unit 40 and supplies power to thisunit through transformers 62, 66 and 69 to set the unit in condition foroperation. The high voltage applied across the photomultiplier tube ison the order of 1000 volts.

The reel on the motor vehicle is now turned to bring up the probe unitthrough the drilled hole at a predetermined rate. As the probe unitpasses through the various deposits, the amount and energy of anyradiation emitted therefrom will be registered in the following manner:The radiation impinging on the scintillation Crystal will result inpositive output pulses from the last dynode 28 of the photomultipliertube 25. These pulses are applied to the control electrode of the tube34.

Tubes 34 and 38 serve to amplify the pulses and apply them to the poweramplifier tube 44. These output pulses are transformer-coupled from thewinding 52 of output pulse transformer 53 to the coaxial cable I6. Theoutput pulses appearing on coaxial cable I6 are applied throughtransformer ||1 and network |22 to the control electrode of the tube l2Where they are further amplified and applied through terminals |33 and|34 to the control electrode of the first univibrator tube |36.

The operation of the univibrator circuit comprising tubes |36 and |44results in a rectangular positive pulse of predetermined height andwidth appearing at the plate of the tube |36 and applied to the controlelectrode of the cathode follower tube |56. taken from the cathode oftube |56 and applied on conductor I8 to the tapped condenser bank |66where the amplitude of these pulses may be varied further.

The output of the tapped condenser bank |66 is applied to the ratemetercircuit tube |12 resulting in a voltage proportional to the repetitionrate of the arriving rectangular pulses. This proportional voltage isapplied to the vacuumtube voltmeter on conductor I9 and indicated onmeter |91 or a recording voltmeter attached to connecting jack |99. Ifthe paper on the recording voltmeter unit is wound at a rateproportional to the rate of rewinding the unwound cable, a permanentrecord is obtained as to the radioactivity of the deposit at anyposition in the drilled hole.

The scale of this ratemeter can be varied independently of thepredetermined height and Width of the positive rectangular pulses. Asdemonstrated above, variation of the tap |61 will permit choice ofdifferent sized condensers from condenser bank |66 which in turn willvary the amplitude of the pulses applied to the diode |12. This providesa scale changing feature which does not require resetting the timeconstants of the univibrator circuit. This feature is These rectangularpulses are 10 particularly important when wide variations of particleenergy levels are encountered in a drill hole.

The ammeter |91 may be used to give instantaneous indication of theradioactivity of the deposit through which the probe unit is passing.The permanent record of the recording voltmeter is available for furtherstudy of the determination of the extent and energy level of anyradioactive material in the vicinity of the drilled hole.

Instead of adding additional stages of amplification between terminals|33 and |34 of amplifier 30, it is possible to connect a conventionalpulse height analyzer to determine the energy level of the detectedradiation. Also, the tapped resistor bank |24 of network |22 inamplifier 30 may be varied to exclude all pulses derived from radiationbelow a desired energy range. For example, if it is desired to detectonly gamma rays above five hundred thousand electron volts in energy,resistor bank |24 is set so that pulses derived from radiation belowthis energy range will not have sufiicient amplitude to initiate theunivibrator action.

Therefore, the only rectangular pulses developed in the univibrator willbe due to gamma rays above ve hundred thousand electron volts. Thisdiscriminator action is particularly useful if uranium is beingprospected and it is suspected that other radioactive deposits such aspotassium are in the vicinity. The radiation emanating from the depositsother than uranium would then be too low to be recorded in the surfaceunit. Such energy discrimination is not possible with con'- ventionalGeiger tube probes.

Referring now to Fig. 4, the physical assembly of the various circuitsof the probe unit will be illustrated to indicate the improvedstructural features of the instrument and its design for ease inmaintenance and replacement of component parts, The probe unit is shownin Fig. 4 with its cylindrical elongated housing removed to reveal theinterior constructional details. At the left of the gure is shown thescintillation crystal 200 enclosed in an hermetically sealed mounting toavoid the deleterious effects of moisture and foreign matter.

The crystal 200 is shown mounted adjacent the photosensitive face of thephotomultiplier tube 25 by means of an annular ring 26|. Thephotomultiplier tube 25 is mounted in turn on an openended cylindricalbase 262 with two annular mounting rings 203 and 204. The cylindricalbase 202 is used to hold the voltage dropping resistors 26 required foruse with tube 25. After the resistors have been placed within base 202,the base may be filled with a potting compound to permanently positionthe resistors and avoid effects of moisture.

Attached to the base 202 is a rigid assembly mounted on a plurality ofrods 206. For purposes of clarity rods 206 are not shown extended to thebase 202. Aiixed in spaced relation to the rods 206 are sevenelectrically insulated wafers 201. Wafers 201 are maintained in theproper spaced relation by means of spacers 208. Shown mounted on thefirst wafer 201 nearest the openended base 202 is the preamplifier tube34. Amplifier tubes 34 and 38 may be enclosed in one tube envelope ofthe conventional twin triode type and represented by the tube envelope34.

The power amplifying tube 44 is also mounted on this wafer behind tube34 but is not visible in this view. On the second electrically insulatedwafer 267 is mounted the output pulse transformer 53. Next in line is afirst electrically conductive plate 209 which is attached to thegrounded rods 2&6 and serves to electrically shield the preamplifier andpower ampliner unit from the following circuits. Mounted on the thirdand fourth wafers 2o? is the high voltage power supply unit. Connectedbetween the high voltage power supply unit and the cylindrical base is ashielded cable 2G5 going to the photomultiplier tube 25.

Shown in the gure is the high voltage rectifier tube 'I9 and transformer62. A second electrically conductive plate 2H "separates the highvoltage supply unit from the remaining circuits. Mounted on the next twowafers is the low voltage power supply consisting of tube 92, condensersSl and S8, and low voltage transformer 69. On the last wafer is mountedthe filament transformer 66. Next to the filament transformer is mounteda coaxial Connector 2l? for attachment to the coaxial cable i6.

The use of the rigid assembly attached to the photomultiplier tube baseresults in an unusually compact, easily maintainable unit. lf it isdesired to replace any part of the unit, new wafers Zi'i can be wired upwith the proper component parts and merely substituted for a defectivecomponent part. The electrically conductive shields 209 and 2li serve tocompletely isolate the various operating circuits of the probe unit andyet not unnecessarily lengthen the overall probe unit.

A satisfactory probe unit has been constructed in this manner, so thatthe assembled unit has an outer diameter of two inches and an overalllength of approximately two and one-half feet. By use of this type ofconstruction, the load on the cable is also considerably lessened,serving to prevent wear and tear on the cable. Other advantages will beapparent from the foregoing description.

^ If the use of rotating machinery is undesirable from the viewpoint ofunwanted noise and interference, the generator l of the circuit may bereplaced with a conventional 40G-cycle electronic power supply which canbe easily operated from the storage battery of the motor vehicle.

Although the unit has been described as being particularly adaptable foruse with drill hole boring apparatus, it has been found useful in makingpreliminary aerial surveys of deposits. For example, to determinepromising locations for starting the drill hole boring apparatus, theprobe unit may be mounted on the Wing of a plane and the surface unitmounted within the fuselage. Due to the efficient streamlined design ofthe probe unit,

` the apparatus is very suitable for this work. With the fast responsetime of the instrument, the speed of the plane is no handicap indetermining the approximate location of radioactive areas. Surveys ofthis type may also be made while the probe unit is mounted on the motorvehicle.

This fast response time of the circuits of the apparatus is useful inthe drill hole radioactive logging operation because the rate ofascendancy of the probe unit can be very fast and serves to lessen thetime taken by this operation while still maintaining the high measuringaccuracy of the instrument. For normal usage the probe unit of theinstrument can be raised at a rate of thirty feet per minute which is vetimes as fast as conventional instruments. It is possible to obtainaccurate logging even with faster ascendancy rates.

The instrument is particularly adapted for use in uranium prospecting.One crystal which has been found satisfactory for measuring theaccompanying gamma radiation has been the thalliumactivated sodiumiodide crystal. However, it is apparent that the instrument can be usedfor measuring all types of nuclear radiations.

While the salient features of this invention have been described indetail with respect to one embodiment, it will ofcourse be apparent thatnumerous modifications may be made within the spirit and scope of thisinvention and it is therefore not desired to limit the invention to theexact details shown, except insofar as they may be defined in thefollowing claims.

We claim:

1. Nuclear radiation measuring apparatus, for use with drill hole boringapparatus, which comprises in combination a subsurface unit, a surfaceunit, a coaxial cable connecting said units, said subsurface unitincluding a scintillation crystal responsive to the radiation impingingthereon and emitting photons in proportion to the amount and energy ofSaid radiation, means responsive to said photons for developing voltagepulses proportional to the number of said photons, means for amplifyingsaid voltage pulses, power amplifying means for developing output pulsesfor transmission along said cable to said surface unit, said poweramplifying means having a transformer-coupled output and a condenser inseries with the primary of said output transformer, said surface unitincluding amplifying means for said output pulses, said amplifying meanshaving a transformercoupled input, a condenser in series with theprimary of said input transformer, oscillating means responsive to saidamplified output pulses and developing one rectangular pulse ofpredetermined amplitude and width for every amplified output pulse,means for varying the amplitude of said rectangular pulses, a rate-metercircuit responsive to said rectangular pulses for indicating the rate ofarrival thereof, and power supply means for supplying operating voltagesto said subsurface unit along said coaxial -cable and simultaneously tosaid surface unit.

2. The apparatus ofv claim 1 wherein said scintillation crystal isthallium-activated sodium iodide and said means responsive to saidphotons is a photomultiplier tube.

3. Nuclear radiation measuring apparatus, for use with drill hole boringapparatus, which comprises in combination a subsurface unit, a surfaceunit, a coaxial cable connecting said units, said subsurface unitincluding means responsive t0 the radiation impinging thereon, means fordeveloping voltage pulses proportional to the amount and energy of saidradiation impinging upon said first means, means for amplifying saidvoltage pulses and developing output pulses for transmission along saidcable to said surface unit, said surface unit including means foramplifying said output pulses, a univibrator responsive to saidamplified output pulses and developing one rectangular pulse ofpredetermined amplitude and width for every amplified output pulse, acathode follower connected to the output of said univibrator, a variablecondenser having one of its ends connected to the cathode of saidcathode follower and its other end connected to a ratemeter circuit,said ratemeter circuit including a double diode with a variableresistor-condenser network in the output thereof, a bridge-typevacuum-tube voltmeter for recording the rate of arrival of saidrectangular pulses and power supply means for supplying operatingvoltages to said subsurface unit along said coaxial cable andsimultaneously to said surface unit.

4. Nuclear radiation measuring apparatus, for use with drill hole boringapparatus, which comprises in combination a subsurface unit, a surfaceunit, a coaxial cable connecting said units, said subsurface unitincluding a scintillation crystal responsive to the radiation impingingthereon and emitting photons in proportion to the amount and energy ofsaid radiation, means responsive to said photons for developing voltagepulses proportional to the number of said photons, means for amplifyingsaid voltage pulses, Ipower amplifying means for developing outputpulses for transmission along said cable to said surface unit, saidpower amplifying means having a transformercoupled output and acondenser in series with the primary of the output transformer, saidsurface unit including amplifying means for said output pulses, saidamplifying means having a transformer-coupled input, a condenser inseries with the primary of said input transformer, oscillating meansresponsive to said amplified output pulses and developing onerectangular pulse of predetermined amplitude and width for everyamplified output pulse, a cathode follower vconnected to the output ofsaid oscillating means, a variable condenser having one of its endsconnected to the cathode of said cathode follower and its other endconnected to a ratemeter circuit, said ratemeter circuit including adoubled iode with a variable resistor-condenser network in the outputthereof, a bridge-type vacuum-tube voltmeter for recording the rate ofarrival of said rectangular pulses, a generator for supplying analternating current voltage to said subsurface unit, a low voltagesupply in said subsurface unit responsive to said alternating currentVoltage for developing a low voltage tube operating potential, said lowvoltage supply having an input transformer coupled to said coaxialcable, a high voltage supply in said subsurface unit responsive to saidalternating current voltage and developing a regulated high voltage foroperation of said photon-responsive means, said high voltage supplyhaving an input transformer coupled to said coaxial cable and powersupply means for said surface unit.

5. For use in drill hole radioactive logging apparatus wherein a surfaceunit supplies power to and measures the output pulses from a subsurfaceprobe unit, an improved probe unit comprising a rigid assembly suitablefor disposition in an elongated housing, and including a photomultipliertube mounted on an open-ended cylindrical base, a plurality of resistorsconnected to said multiplier tube and mounted within said cylindricalbase, a scintillation crystal affixed adjacent the photosensitive faceof said photomultiplier tube, a plurality of rods terminating at one endin said cylindrical base and at the other end in 'a coaxial jack, aplurality of electrically insulated parallel wafers affixed to said rodsin spaced relation between said cylindrical base and said coaxial jack,a preamplifier tube and a power amplifier tube mounted on the first ofsaid wafers nearest said cylindrical base, said preamplifier tube being'connected to the output of said photomultiplier tubs and said poweramplifier tube being connected to the output of said preamplifier tube,a pulse transformer mounted on the second said wafer and connectedbetween the output of said power amplifier tube and said coaxial jack, afirst electrically conductive plate affixed to said rods andelectrically isolating the circuits mounted on said first and secondwafers from the remaining circuits, a high voltage power supply mountedbetween the third and fourth of said wafers, a high voltage transformermounted on the fourth of said wafers, and connecte-.fl between the inputof said high voltage power supply and said coaxial jack, the output ofsaid high voltage power supply being connected to said photomultipliertube, a second electrically conductive plate to said rods andelectrically isolating the high voltage supply from the other circuitsmounted in the probe unit, a low voltage supply mounted on the fifth andsixth of said wafers, the input of said low voltage supply beingconnected te said coaxial jack and the output being connected to saidpreamplifier1 and power amplifier tubes, a filament transformer mountedon the seventh of said wafers and a third electrically conductive plateaffixed to said rods and mounted on said coaxial jack.

6. Nuclear radiation measuring apparatus for use with drill hole boringapparatus, which comprises in combination a subsurface unit, a surfaceunit, a coaxial cable connecting said units, said subsurface unitincluding a rigid assembly adapted for removable disposition in anelongated housing, said assembly including a scintillation crystalmounted on the photosensitive face of a photomultiplier tube, saidscintillation crystal being responsive to the radiation impingingthereon and emitting photons in proportion to the amount and energy ofsaid radiation, said photomultiplier tube being responsive to saidphotons and developing voltage pulses proportional to the number of saidphotons, a plurality of rods terminating at one end in a cylindricalbase and at the other end in a coaxial jack, a plurality of electricallyinsulated parallel wafers afxed to said rods in spaced relation betweensaid cylindrical base and said coaxial jack, means for amplifying saidproportional voltage pulses and power amplifying means both mounted onthe first of said wafers nearest said cylindrical base, said poweramplifying means being responsive to the amplified voltage pulses anddeveloping output pulses for transmission along said cable to saidsurface unit, said subsurface unit further including electricallyconductive plates to isolate the various circuits thereof from oneanother, the circuits of said subsurface unit including a pulsetransformer mounted on the second said wafer, a high voltage powersupply mounted between the third and fourth of said wafers, a highvoltage transformer mounted on the fourth of said wafers, a low voltagepower supply mounted on the fifth and sixth of said wafers and afilament transformer mounted on the seventh of said wafers, and saidsurface unit including means for amplifying said output pulses,oscillating means responsive to said amplied output pulses anddeveloping one rectangular pulse of predetermined amplitude and widthfor every amplified output pulse, means for varying the amplitude ofsaid rectangular pulses, a ratemeter circuit responsive to saidrectangular pulses for indicating the rate of arrival thereof and powersupply means for supplying an alternating current voltage to saidsubsurface unit along said coaxial cable and simultaneously supplyingoperating voltages to said surface unit.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,481,014 Herzog Sept. 9, 1949 2,534,932 Sun Dec. 19, 19502,550,106 Coltman et al Apr. 24, 1951 2,648,012 Scherbatskoy Aug, 4,1953

